An ultrasound diagnostic apparatus is designed to transmit an ultrasound signal inside an object to be examined and to obtain information available in diagnosis, e.g. a tomographic image, on the basis of a received signal, including an echo signal of the transmitted signal. It has been reported that, in the technique of displaying a tomographic image, a high-contrast image can be obtained by imaging a harmonic component (e.g. frequency 2f0 or 3f0) as opposed to a fundamental component (frequency f0) of the transmitted signal. An imaging method of this type is referred to as “Tissue Harmonic Imaging.”
The above-described harmonic component is generated due to nonlinear distortion occurring mainly when the ultrasound is propagated inside the object. That is, a signal of the ultrasound irradiated to the inside of living body is distorted during propagation in tissue due to a nonlinear response of the tissue, and the harmonic component is increased. As a result, the echo signal includes, e.g. a double frequency 2f0 and a triple frequency 3f0 of a fundamental component f0.
In tissue harmonic imaging, it is important how to extract an echo of a strong harmonic component. The conventionally reported methods of tissue harmonic imaging include one referred to as a “filtering technique”, for example.
This technique is designed to extract the harmonic component of e.g. 2f0 from a received signal using a band pass filter with a central frequency of e.g. 2f0. Another example is a method referred to as a “pulse inversion technique,” which is designed to emphasize the second harmonic component by transmitting first and second waveforms having a mutually alternated polarity at predetermined time intervals, and phasing and adding the echo signals thereof to cancel the fundamental component. Further, for example, Japanese Unexamined Patent Publication No. 2002-34976 discloses a technique of extracting a harmonic component through a filter from received signals obtained from transmitted signals having two different center frequencies, and widening the band of the harmonic component by combining those extracted signals, thus improving the resolution and enhancing the signal strength in a beam depth direction, as well as suppressing generation of motion artifacts. However, because the frequency of the harmonic component in an ultrasound signal is higher than that of the fundamental component, the harmonic component is sensitive to attenuation of the signal during propagation. Accordingly, the degree of an echo signal received from a deep portion, i.e. the penetration, is undesirable. Meanwhile, when the center frequency f0 of the fundamental component is lowered, the echo signal is scarcely affected by the attenuation, whereby the penetration can be improved. However, the resolution is deteriorated, as is generally known.